Video disc lubricants

ABSTRACT

Molecular distillation of methyl alkyl siloxanes of the formula ##STR1## wherein R 1  and R 2  are alkyl groups of 4-20 carbon atoms and m and p are integers to produce a distillate having a molecular weight fraction wherein the sum of m and p is about 4, produces an improved lubricant for the video disc, a lubricant that is stable to long term storage and to wide variations in temperature and relative humidity.

This is a division of application Ser. No. 937,819, filed Aug. 29, 1978,now abandoned.

This invention relates to improved lubricants for video discs. Moreparticularly, this invention relates to the purification andfractionation of methyl alkyl siloxane lubricants which give improvedperformance as video disc lubricants.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,833,408, herein incorporated by reference, describes theapplication of methyl alkyl siloxane compositions as lubricants forconductive video discs comprising a molded plastic disc having audio andvideo information in the form of geometric variations in a spiralgroove. These discs are coated first with a conductive material whichacts as a first electrode, then with a dielectric layer and a finallayer of lubricant. A metal tipped stylus acts as a second electrode ofa capacitor and the information signals are monitored by the styluswhich notes changes in capacitance between the stylus and the discsurfaces as the information, in the form of depressions, passes beneaththe stylus.

Further developments in this system have produced a video disc which ismade of a conductive plastic material, e.g., a PVC copolymer resincontaining sufficient amounts of conductive carbon particles so that thedisc can provide capacitance readout, while the plastic resin surroundsthe carbon particles providing a dielectric surface layer on theconductive particles. This development has eliminated the need forseparate coatings of metal and dielectric on the plastic disc.

The stylus, formerly made of metallized sapphire, has also been improvedso that metallized diamond can be used. Diamond is a harder, longerwearing material than sapphire, and also requires good lubrication ofthe disc surface.

Video discs are also being developed which do not require a conductivesurface or a grooved surface, the stylus being maintained insynchronization with the information pattern track by means ofelectrical signals rather than the groove walls.

These changes in the materials used for the video disc and stylus havesomewhat changed the requirements for the lubricant and in certainrespects the commercially available methyl alkyl siloxane lubricant isnow unsatisfactory. One commercially available methyl alkyl siloxanecomposition has the formula ##STR2## wherein R is a straight chain decylgroup, and n is an integer. This material has a molecular weight ofabout 1500 and a viscosity of about 50 centistokes and is sold byGeneral Electric Company as SF-1147. This composition also containsabout 1.5 percent of antioxidant compounds.

When the present video discs molded from carbon-loaded PVC compositionsare spray coated with the SF-1147 lubricant and played back with adiamond stylus, the playback performance and stability are less thandesirable. Thus an attempt was made to improve the performance of thisclass of lubricant and the present invention resulted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a molecular distillation apparatususeful in preparing compositions of the invention.

FIG. 2 is a graph which shows the variation in amount of distillateversus temperature.

FIG. 3 shows the infrared spectrum of distilled and undistilled methylalkyl siloxane compositions at 2150 cm⁻¹.

SUMMARY OF THE INVENTION

We have found that the methyl alkyl siloxanes having the formula##STR3## wherein R₁ and R₂ are alkyl groups of 4-20 carbon atoms, x isan integer of 2-4 and y is an integer of 0-2 and wherein the sum of xand y is 4 or less, have improved lubricity for video disc applicationsand improved stability to high temperatures, high and low humidityconditions and improved resistance to ageing.

DETAILED DESCRIPTION OF THE INVENTION

The methyl alkyl siloxane compositions of the invention have adistillation temperature of about 100°-205° C. at a pressure of 10⁻⁵torr, have a molecular weight range of about 500-800 and apolydispersity, defined as the weight average molecular weight dividedby the number average molecular weight, of less than about 1.07. Thefractions useful herein can be obtained by molecular distillation of amixed siloxane starting material of the formula ##STR4## wherein R₁ andR₂ are as defined above and m and p are integers, under high vacuum lowtemperature conditions, to obtain a narrow molecular weight fraction.

Molecular distillation is known and is described in "MolecularDistillation of Thermally Supersensitive Liquids", Frank et al,Analytical Chemistry, Vol. 36, No. 11, October 1964. The method can beexplained in detail by referring to FIG. 1 which is a schematic view ofa molecular still 10. The material to be distilled is fed to feedreservoir 12 which is evacuated by diffusion pump 14 through line 16which passes through a liquid nitrogen trap 18. The feed reservoir 12 isconnected to a distillation chamber 20 containing a glass column 22which has a spiral glass winding 24 wrapped around it and a receptacle26 at one end to contain hot fluid. The distillation chamber 20 alsocontains two additional receptacles, a distilland receptacle 28 and adistillate receptacle 30. A feed tube 32 allows material from the feedreservoir 12 to flow to the distillation chamber 20 through valve 34. Ahigh vacuum is maintained in the distillation chamber 20 through line36, which also leads to the diffusion pump 14. During operation, thesystem is evacuated to an initial pressure of about 10³¹ 7 torr. Boilingwater is maintained in the receptacle 26 and rises through column 22 toheat the said column and the glass winding 24 to a temperature of about100° C. The control valve 34 is opened to allow a small stream of thematerial to be distilled to drop onto the heated column 22 from the feedreservoir 12. The material is spread out by the wrapped coil 24 to forma smooth, thin film which is turned over periodically as it passes alongthe winding 24, enabling the more volatile constituents to evaporate.The material which vaporizes at 100° C. or less evaporates from theheated column 22 and is condensed on the wall of the distillation column20, flows down the wall and is collected in distillate receptacle 30.The remaining material is collected in distilland receptacle 28. Toseparate out the materal which boils above 100° and below 205° C., forexample, the receptacle 26 is filled with benzyl alcohol, which boils at205° C., and the distillate from the first fractionation is fed to thefeed reservoir 12 and the distillation repeated. The desired distillatecollects in distillate receptacle 30 as before and the higher boiling,higher molecular weight fractions are collected in distilland receptacle28. By using this method of fractionation, the liquid bulk materialitself is not heated, but only contacts a heated column where itundergoes a collision-free evaporation at minimum temperatures with theleast damage to thermally sensitive molecules.

We have found that molecular distillation of the present methyl alkylsiloxane feedstock not only fractionates the feedstock, but alsopurifies it, both by removing all antioxidant compounds and chromophoresand silicon hydrides as well, thereby providing a much improvedlubricant for the video disc. This purification result was highlyunexpected and surprising and is highly desirable, since hydrides arechemically active and can produce gel formation which would lead todropouts and increased noise on playback of the video disc.

The presence of aromatics such as metal-aromatic complex chromophoresand antioxidants is also highly undesirable since such compounds arereactive to the video disc surface and contribute to a deterioration ofthe playback quality with time. The antioxidants generally added to themethyl alkyl siloxane compounds are in the form of both freeantioxidants and chemically bound antioxidants. The free antioxidant hasthe formula ##STR5## and the chemically bound antioxidant has theformula ##STR6## Since the concentration of unbound antioxidant variesfrom batch to batch, leading to nonuniformities in playback performanceof the video disc, the unbound antioxidant can be removed by dissolvingin acetone and separating the acetone layer from the methyl alkylsiloxane. However, the presence of the chemically bound antioxidantrenders the video disc unplayable, often in a matter of weeks, and thusthe presence of aromatic impurities and antioxidant compounds cannot betolerated at all in the lubricant for the present video discs.

That the present molecular distillation procedure would remove siliconhydrides, particularly to the distilland portion or high molecularweight fraction, was highly unexpected, since silicon hydrides aregenerally low boiling compounds. It is believed that even atcomparatively low temperatures, the hydrides present are reacting withthe methyl alkyl siloxane compounds, either to produce higher molecularweight fractions or to produce gelling. This also contributes to theageing process by using up the lower molecular weight fractions andproducing gels which lead to a massive dropout of information duringplayback of the video disc. This result was highly unexpected.

The fractionated methyl alkyl siloxanes of the invention can bedissolved in a suitable solvent, such as heptane or isopropanol or othersolvent which is inert with respect to the disc surface, typically at aloading of from about 0.2 to 2.0 percent by weight of the solution ofthe methyl alkyl siloxane. The solution is then sprayed onto the discsurface to form a siloxane film about 200-300 angstroms thick. Thelubricant film can also be applied by evaporation.

The present lubricant fractions do not age, are stable with respect toatmospheric effects and provide high uniformity and reproducibility forthe video disc. In addition, the fractions provide excellent lubricityas measured by low stylus and disc wear, and improved initial playbackperformance.

The invention will be further illustrated by the following Examples butthe invention is not to be limited to the details described therein. Inthe Examples, percent is by weight unless otherwise noted.

EXAMPLE 1

A methyl alkyl siloxane feedstock having the formula ##STR7## wherein nis an integer from 1 to about 8, having a viscosity of 49.4 centistokesat 22° C., a number average molecular weight of 1239 and a weightaverage molecular weight of 1557, thus a polydispersity of 1.26, and arefractive index of 1.4463 was molecularly distilled at an averagepressure of 10⁻⁵ torr in a distillation apparatus according to FIG. 1.

FIG. 2 shows the amount of distillate as a percent by weight of thefeedstock versus temperature.

The fraction which distilled at from 100°-205° C. was collected.

The distillate had a viscosity of about 14 centistokes at 25° C.; aweight average molecular weight of 660, which corresponds to acomposition of the general formula (1) wherein y is 0, x is 3 and R₁ isdecyl, a dispersity of 1.06 and a refractive index of 1.435.

The color of the distillate was measured by determining optical densityat various wavelengths for a 1.0 millimeter thick sample and compared tothe feedstock color and the distilland color after moleculardistillation. The data are summarized below in Table I.

                  TABLE I                                                         ______________________________________                                                     Optical Density                                                  Sample         500 nm     400 nm  300 nm                                      ______________________________________                                        Feedstock      0.060      0.125   1.60                                        Distillate 100-205° C.                                                                0.035      0.055   1.40                                        Distilland     0.080      0.17    1.80                                        ______________________________________                                    

Thus the distillate contains less color-forming material than either thefeedstock or the distilland at all wavelengths tested.

The hydride content of the distillate was also greatly reduced from thatof the feedstock. As shown by infrared absorption, at a wavelength of2150 cm⁻¹ (SiH) for a 1.0 millimeter thick sample, the feedstockcontained about 5-12 cm⁻¹ of silicon hydride. The distillate containedless than 0.1 cm⁻¹ hydride. The infrared spectra for the feedstock isshown in Curve 1 and the distillate spectra is shown in Curve 2 of FIG.3.

Thus molecular distillation both fractionates and purifies the methylalkyl siloxane feedstock.

EXAMPLE 2

The feedstock of Example 1 was molecularly distilled into 5 fractionsusing appropriate heating fluids and various measurements were taken.The data are summarized in Table II below, wherein fraction I wascollected at 40° C., fraction II was collected at 100° C., fraction IIIwas collected at 153° C., fraction IV was collected at 211° C. andfraction V was collected at 239° C.

                  TABLE II                                                        ______________________________________                                                                                   Refrac-                                     Density, Viscosity,         Poly- tive                                        gm./ml.  cs at              disper-                                                                             Index,                             Fractions                                                                              at 22° C.                                                                       22° C.                                                                          --M.sub.n                                                                          --M.sub.w                                                                          sity  22° C.                      ______________________________________                                        Feedstock                                                                              0.868    49.4     1239 1557 1.26  1.4463                             Distilland I                                                                           0.868    49.2     1216 1507 1.24  1.4452                             Distilland II                                                                          0.869    53.0     1323 1654 1.25  1.4453                             Distilland III                                                                         0.869    55.9     1450 1794 1.24  1.4455                             Distilland IV                                                                          0.874    63.2     1525 1872 1.23  1.4472                             Distilland V                                                                           0.874    73.5     2229 2700 1.21  --                                 Distillate II                                                                          0.849    10.6      500  526 1.05  1.4353                             Distillate III                                                                         0.852    15.5      644  690 1.07  1.4363                             Distillate IV                                                                          0.862    22.7      800  836 1.05  1.4385                             Distillate V                                                                           0.863    27.9     1248 1304 1.04  --                                 ______________________________________                                    

The above table shows that methyl alkyl siloxanes can be fractionatedinto narrow molecular weight fractions by the molecular distillationtechnique.

EXAMPLE 3

A series of video discs were tested by applying various lubricantcompositions and fractions to the disc by evaporation or sprayingtechniques and playing back with a titanium metallized diamond stylus.

Lubricant A was a methyl decyl siloxane of the formula ##STR8## with amolecular weight range as described in Example 1, free of anyantioxidant and was applied by vacuum evaporation; lubricant B was thesame lubricant applied by spraying from a 1 percent solution in heptane;and lubricant C was the fraction molecularly distilled at 100°-205° C.,also applied by spraying as above. The lubricant layers were about 200angstroms thick.

The quality of the video discs during playback was measured bydetermining the carrier distress time, which is measured by adding theamount of time, in seconds (but discounting intervals of less than 10microseconds) during total disc playback when the R.F. output of theplayer arm is less than 150 millivolts peak to peak, and the time whenthe R.F. output gave above 8.6 megahertz or below 3.1 megahertz infrequency, indicating a defect. Such defects are noted by the viewer asdropouts. The pass criterion for a video disc is a maximum of 3 secondsin 30 minutes of playback time, and good video quality is considered tobe less than 0.3 second in 30 minutes of playback.

Twelve discs having the above lubricants were tested and the percentageof discs which passed each standard was calculated. The data aresummarized below in Table III.

                  TABLE III                                                       ______________________________________                                                  Carrier Distress Time, sec./30 min.                                             % ≦                                                                            % ≦                                                                              % ≦                                                                          % ≦                                Lubricant   0.3 sec.                                                                              1 sec.    3 sec.                                                                              6 sec.                                    ______________________________________                                        A           41.7    58.4      83.4  100                                       B           66.7    75        91.7  91.7                                      C           91.7    91.7      91.7  91.7                                      ______________________________________                                    

Stylus and disc surface wear tests were made on the above twelve discsby carrying out 35 playbacks on the same 20 minute band of each disc andmeasuring styli wear and examining the disc surfaces before and afterthe test for gross, visible wear and significant surface damage visibleto the eye and/or under a microscope. The data are summarized below inTable IV.

                  TABLE IV                                                        ______________________________________                                                                % of discs                                                                    with gross wear                                                    Stylus wear,                                                                             and significant                                       Lubricant    μ.sup.3 /hour                                                                         surface damage                                        ______________________________________                                        A            0.003      25                                                    B            0.014      33.3                                                  C            0.003      8.4                                                   ______________________________________                                    

EXAMPLE 4

The effect of low humidity on a lubricant fraction as in Example 1, wastested by measuring the carrier distress time of 12 video discs underambient conditions (76° F. and 18 percent relative humidity, hereinafterRH). All of the discs had carrier distress times of less than 0.3seconds in 30 minutes of playback. They were then stored at 66° F. and 6percent RH for 72 hours and played again. All of the discs played welland had a carrier distress of less than 0.3 second in 30 minutes ofplayback.

When a feedstock lubricant was applied to video discs and tested asabove, 42 percent of the discs exhibited carrier distress times of lessthan 0.3 second and 75 percent of the discs had carrier distress timesof less than 3 seconds in 30 minutes of playback. This was unchangedafter low humidity storage.

EXAMPLE 5

The effect of low temperature on the present lubricant was tested bymeasuring carrier distress times of 12 video discs coated with thefractionated lubricant as in Example 1 under ambient conditions (76° F.,20 percent RH). All of the discs had a carrier distress time of lessthan 0.3 second in 30 minutes of playback. They were then stored in alow temperature chamber at 59°-62° F. and 25 percent RH for 72 hours andimmediately played again. All of the discs exhibited good performancebefore and after storage and all still had a carrier distress time ofless than 0.3 seconds in 30 minutes of playback.

When a feedstock lubricant control was applied and tested as above, only33 percent of the discs had carrier distress time of less than 0.3second and 75 percent of the discs had carrier distress times of lessthan 3 seconds in 30 minutes of playback. These results were unchangedafter low temperature storage.

EXAMPLE 6

The effect of high temperature and high humidity on the presentlubricant was tested by measuring carrier distress time of 12 videodiscs coated with the fractionated lubricant as in Example 1 afterstorage in a chamber at 95° F. and 75 percent RH for 24 hours. Carrierdistress time results are summarized below in Table V.

                  TABLE V                                                         ______________________________________                                                  Carrier Distress Time, sec./30 min.                                             % ≦                                                                            % ≦                                                                              % ≦                                                                          % ≦                                Lubricant   0.3 sec.                                                                              1 sec.    3 sec.                                                                              6 sec.                                    ______________________________________                                        A           16.7    66.7      100   100                                       B           16.7    50        83.3  100                                       C           33.3    83.3      100   100                                       ______________________________________                                    

EXAMPLE 7

A mixed methyl alkyl siloxane of the formula ##STR9## wherein m and pare integers of 2-7 was prepared and molecularly distilled to collectthe fraction boiling from 100°-205° C. This lubricant fraction wassprayed from a 1 percent heptane solution onto a video disc as inExample 1. Playback data initially and after storage at high temperatureand high humidity conditions (95° F. and 75 percent RH) are summarizedbelow in Table VI.

                  TABLE VI                                                        ______________________________________                                                  Carrier Distress Time, sec./30 min.                                           % ≦                                                                          % ≦                                                                              % ≦                                                                            % ≦                                            0.3 sec.                                                                            1 sec.    3 sec.  6 sec.                                      ______________________________________                                        Initial play                                                                              87.5    100       100   100                                       After stor-                                                                   age         12.5    62.5      62.5  87.5                                      ______________________________________                                    

EXAMPLE 8

A series of methyl alkyl siloxanes of the formula ##STR10## wherein Rwas hexyl, octyl and dodecyl respectivly and m was an integer of 2-7were prepared. Each lubricant was sprayed on 8 video discs and thecarrier distress time measured. The data are summarized below in TableVII.

                  TABLE VII                                                       ______________________________________                                                   Carrier Distress Time, sec./30 min.                                             % ≦                                                                            % ≦                                                                              % ≦                                                                          % ≦                               Initial Play 0.3 sec.                                                                              1 sec.    3 sec.                                                                              6 sec.                                   ______________________________________                                        C.sub.6 Lubricant                                                                          87.5    100       100   100                                      C.sub.8 Lubricant                                                                          87.5    87.5      100   100                                      C.sub.12 Lubricant                                                                         87.5    100       100   100                                      ______________________________________                                    

The discs were then stored at 95° F. and 75 percent RH for 72 hours andretested. The carrier distress data obtained are summarized below inTable VIII.

                  TABLE VIII                                                      ______________________________________                                                   Carrier Distress Time, sec./30 min.                                             % ≦                                                                            % ≦                                                                              % ≦                                                                          % ≦                               After Storage                                                                              0.3 sec.                                                                              1 sec.    3 sec.                                                                              6 sec.                                   ______________________________________                                        C.sub.6 Lubricant                                                                          0       62.5      62.5  87.5                                     C.sub.8 Lubricant                                                                          12.5    75        100   100                                      C.sub.12 Lubricant                                                                         12.5    62.5      87.5  100                                      ______________________________________                                    

Stylus wear tests were also made. All of the tests showed stylus wearbelow 0.04μ³ /per hour.

COMPARATIVE EXAMPLE

A commercially available methyl alkyl siloxane composition sold asSF-1147 was treated to remove free antioxidant as follows: a lubricantwas shaken with an equal volume of acetone twice and the layersseparated. The free antioxidant is dissolved in the acetone layers alongwith some low molecular weight silicone oils. The siloxane layer wascollected, stripped at 80° C. under vacuum and evacuated at 10millimeters of mercury at 80° C. to remove all free antioxidant, whileleaving the chemically bound antioxidant and the molecular weightdistribution of the feedstock intact. This composition was thenmolecularly distilled as in Example 1 and the fraction distillingbetween 100°-205° C. was collected.

The lubricant fraction was applied by spray coating from heptane onto avideo disc. The initial playback data was good; 84 percent of 12 discshad a carrier distress time of 0.3 second in 30 minutes of playback orless. The discs were then stored for 1 week under ambient conditions.Very high carrier distress times, about 100 seconds up to about 1000seconds in 30 minutes of playback, were noted on all discs. Thelubricant was found to have been removed in places, indicating ageingand lack of compatibility with the present disc system when chemicallybound antioxidant is present in the lubricant.

We claim:
 1. A lubricant composition for a video disc comprising amethyl alkyl siloxane fraction of the formula ##STR11## wherein R₁ andR₂ are alkyl groups of 4-20 carbon atoms, x is an integer of 2-4, y isan integer of 0-2 and wherein the sum of x+y is 4 or less, said fractionhaving a distillation temperature of about 100°-205° C. at a pressure of10⁻⁵ torr and wherein said composition is free of antioxidant.
 2. Thelubricant composition according to claim 1 wherein y is 0 and R₁ isdecyl.
 3. The lubricant composition according to claim 1 wherein y is 0and R₁ is octyl.
 4. The lubricant composition according to claim 1wherein y is 0 and R₁ is hexyl.
 5. The lubricant composition accordingto claim 1 wherein y is 0 and R₁ is dodecyl.
 6. The lubricantcomposition according to claim 1 wherein R₁ is butyl and R₂ is octyl. 7.The lubricant composition according to claim 1 wherein R₁ and R₂ arealkyl of 4-12 carbon atoms.
 8. A method of purifying and fractionating acomposition of the formula ##STR12## wherein R₁ and R₂ are alkyl groupsof 4-20 carbon atoms and m and p are integers which comprisesmolecularly distilling said composition and collecting the fractionwhich distills at from about 100°-205° C.